Definition
A small placard mounted near the magnetic compass that shows the compass error (deviation) on various headings, allowing the pilot to correct the indicated heading for magnetic interference caused by the aircraft's own electrical systems and metal components.
Plain English
A little card next to the compass that tells you how far off the compass reads on different headings, so you know what to fly to actually go the direction you want.
Context Anchor
Seen near the magnetic compass during preflight, taxi, takeoff checks, and instrument cockpit checks.
Derivation
Deviation comes from the Latin 'deviare,' meaning 'to turn aside from the way.' The card shows how much the compass needle has been turned aside from true magnetic north by the aircraft itself.
Why Pilots Care
Applying the corrections on the card ensures the magnetic compass provides accurate headings for navigation and instrument flight.
Grounding Statement
The deviation card is there because the compass in that specific aircraft may not point perfectly correctly on every heading.
Intuition Check
Deviation does not mean the pilot has strayed from course here. It means the aircraft’s compass has a known amount of error that must be allowed for.
Example Sentence 1
During preflight, the pilot checked the deviation card and noted that on a heading of 090, the compass needed to be flown at 092 to track east.
Example Sentence 2
During the taxi checklist the pilot verified the deviation card values matched the current compass installation.